Communication systems, such as an IP phone and the like using a packet network, such as a Voice over Internet Protocol (VoIP) network or the like have become widespread. In order to manage communication quality in the packet network, such as the VoIP network or the like, a method in which a communication quality value, such as a loss rate of packets transmitted and received in the packet network or the like is measured, the measured communication quality value is compared with a management reference value that the network administrator has set previously to thereby detect a communication state, and thus the packet network is monitored has been carried out (for example, refer to Japanese Laid-Open Patent Publication No. 2005-159807: Patent Document 1).